“homeopathic”, have been shown to be able to catalyse cross-coupling processes, for
example, employing activated aryl halide substrates under microwave conditions
[32]. Moreover, it has been demonstrated that higher Pd concentrations can lead to
catalyst deactivation, most likely due to aggregation to inactive Pd particles.
Evidence for Leaching of Metal from Heterogeneous Sources
When aggregated, Pd is not necessarily irreversibly bound to the aggregated material. Clear evidence of leaching of catalytically active material from heterogeneous
sources into the solution phase has been documented. Common heterogeneous
catalytic sources have been proven to operate via leached species (e.g. Pd/C,
Pd/SiO 2 and Pd/SiO 2 ) [33–35]. In some cases, such as that reported by Arai et al.,
leaching can be followed by redeposition at the surface, upon completion of the
reaction, allowing for efficient recycling of the heterogeneous catalyst [33]. Reactions that follow this regime can be said to operate via a leaching–redeposition
catalytic mechanism (see Sect. 5.1, for an example in C–H functionalising
chemistry).
Dupont et al. found that PdNPs, generated from a palladacyclic, homogeneous
source could be stabilised and conveniently analysed in imidazolium ionic liquids
[36]. These PdNPs, after successful mediation of a Mizoroki–Heck cross-coupling
reaction, were shown to have released significant amount of Pd in the reaction
solution, due to leaching brought about via oxidative addition on a particle surface.
Rothenberg et al. utilised UV/visible spectroscopic methods to track dynamics of
reduction and concomitant aggregation of heterogeneous Pd from homogeneous
sources [37]. The same group were able to demonstrate leaching of catalytically
active material from relatively large nanoparticles (~ 15 nm) [38]. By design of a
special reactor containing a porous membrane which would only allow passage of
smaller particles, they proved that leached species can be active in cross-coupling
processes. This does not however confirm that the leached species are operating as
homogeneous catalysts, since nanoparticles of this size could theoretically contain
several thousand Pd atoms, and thus, the reaction could still be occurring on a
surface. Baiker et al. used X-ray absorption spectroscopic methods to track, in
situ, the leaching of Pd from a heterogeneous source (Pd/Al 2 O 3 ) under Mizoroki–
Heck reaction conditions. In situ probing also revealed the presence of solutionsuspended, colloidal, PdNPs as well as molecular species [PdBr 4 ]
2À and [Pd 2 Br 6 ]
2À ,
leached into the liquid phase. These results implied that Pd colloids are involved in
catalysis and not merely a catalytic reservoir.
A key message which can be derived from these examples is the evidence that
particulate metal can act as reservoirs, from which leaching of catalytically competent active species can arise.
Additionally, irreversible catalyst leaching can result in loss of expensive metal
catalyst, to either waste streams or contamination of products. The problem of
leaching is of further significance for the pharmaceutical sector, which sets stringent
limits (<10 ppm for oral consumption) on the trace levels of Pd which can contaminate drug products. Thus, there is significant research drive to develop scavenger
systems which can efficiently reclaim Pd lost to reaction solutions [39–40].
Pd Nanoparticles in C–H Activation and Cross-coupling Catalysis
179
example, employing activated aryl halide substrates under microwave conditions
[32]. Moreover, it has been demonstrated that higher Pd concentrations can lead to
catalyst deactivation, most likely due to aggregation to inactive Pd particles.
Evidence for Leaching of Metal from Heterogeneous Sources
When aggregated, Pd is not necessarily irreversibly bound to the aggregated material. Clear evidence of leaching of catalytically active material from heterogeneous
sources into the solution phase has been documented. Common heterogeneous
catalytic sources have been proven to operate via leached species (e.g. Pd/C,
Pd/SiO 2 and Pd/SiO 2 ) [33–35]. In some cases, such as that reported by Arai et al.,
leaching can be followed by redeposition at the surface, upon completion of the
reaction, allowing for efficient recycling of the heterogeneous catalyst [33]. Reactions that follow this regime can be said to operate via a leaching–redeposition
catalytic mechanism (see Sect. 5.1, for an example in C–H functionalising
chemistry).
Dupont et al. found that PdNPs, generated from a palladacyclic, homogeneous
source could be stabilised and conveniently analysed in imidazolium ionic liquids
[36]. These PdNPs, after successful mediation of a Mizoroki–Heck cross-coupling
reaction, were shown to have released significant amount of Pd in the reaction
solution, due to leaching brought about via oxidative addition on a particle surface.
Rothenberg et al. utilised UV/visible spectroscopic methods to track dynamics of
reduction and concomitant aggregation of heterogeneous Pd from homogeneous
sources [37]. The same group were able to demonstrate leaching of catalytically
active material from relatively large nanoparticles (~ 15 nm) [38]. By design of a
special reactor containing a porous membrane which would only allow passage of
smaller particles, they proved that leached species can be active in cross-coupling
processes. This does not however confirm that the leached species are operating as
homogeneous catalysts, since nanoparticles of this size could theoretically contain
several thousand Pd atoms, and thus, the reaction could still be occurring on a
surface. Baiker et al. used X-ray absorption spectroscopic methods to track, in
situ, the leaching of Pd from a heterogeneous source (Pd/Al 2 O 3 ) under Mizoroki–
Heck reaction conditions. In situ probing also revealed the presence of solutionsuspended, colloidal, PdNPs as well as molecular species [PdBr 4 ]
2À and [Pd 2 Br 6 ]
2À ,
leached into the liquid phase. These results implied that Pd colloids are involved in
catalysis and not merely a catalytic reservoir.
A key message which can be derived from these examples is the evidence that
particulate metal can act as reservoirs, from which leaching of catalytically competent active species can arise.
Additionally, irreversible catalyst leaching can result in loss of expensive metal
catalyst, to either waste streams or contamination of products. The problem of
leaching is of further significance for the pharmaceutical sector, which sets stringent
limits (<10 ppm for oral consumption) on the trace levels of Pd which can contaminate drug products. Thus, there is significant research drive to develop scavenger
systems which can efficiently reclaim Pd lost to reaction solutions [39–40].
Pd Nanoparticles in C–H Activation and Cross-coupling Catalysis
179